retrieve width of space character and compute how many spaces are required to fill the gap

spaceWidth.swift

let spaceWidth = { () -> Double in
    let font = self.font!
    var spaceGlyph = CGGlyph()
    var space: UniChar = numericCast((" " as Unicode.Scalar).value)
    _ = CTFontGetGlyphsForCharacters(font, &space, &spaceGlyph, 1)
    return CTFontGetAdvancesForGlyphs(font, .horizontal, &spaceGlyph, nil, 1)
}()
// to be called from func layoutManager(_ layoutManager: NSLayoutManager, shouldSetLineFragmentRect lineFragmentRect: UnsafeMutablePointer<CGRect>, lineFragmentUsedRect: UnsafeMutablePointer<CGRect>, baselineOffset: UnsafeMutablePointer<CGFloat>, in textContainer: NSTextContainer, forGlyphRange glyphRange: NSRange)
let addSpaces = Int(Double(lineFragmentRect.pointee.maxX - lineFragmentUsedRect.pointee.maxX) / spaceWidth)